with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve shows a couple of peaks
with a total duration of about 5 sec. The peak count rate
was ~1800 counts/sec (15-350 keV), at ~0 sec after the trigger.
The XRT began observing the field at 12:59:49.3 UT, 80.7 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 263.15298, -0.94831 which is equivalent
to:

RA(J2000) = 17h 32m 36.72s
Dec(J2000) = -00d 56' 53.9"

with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 97 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. We
cannot determine whether the source is fading at the present time.
A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.88 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 4.1
(+3.92/-2.99) x 10^21 cm^-2 (90% confidence).
UVOT took a finding chart exposure of 150 seconds with the White filter
starting 85 seconds after the BAT trigger. There is a candidate afterglow in
the list of sources generated on-board at

with a 90%-confidence error radius of about 1.10 arc sec. This position is 6.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.03. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.25.
Burst Advocate for this burst is P.A. Evans (pae9 AT star.le.ac.uk).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

GCN Circular #19981
U.Quadri, L.Strabla and R.Girelli report:
We imaged the field of GRB 161004A detected by SWIFT(trigger 715084)
with the robotic telescope of (IAU station 565) Bassano Bresciano
Observatory, Italy. Member of:
AAVSO - American Association of Variable Star Observers.
ISSP - Italian Supernovae Search Project.
UAI/SSV - Unione Astrofili Italiani/sezione stelle variabili.
The observations started 4.90 hour after the GRB trigger, at the end of twilight,
with our Newton telescope D=250 mm F/D=4.8.
Weather conditions were good.
We co-added 3 series of 10 exposures of 60 sec each.

Start T0+ End T0+ Vlim
4.9 hour 5.5 hour 18

We did not found any optical counterpart in the error box of the XRTcandidate.
P.A. Evans (U Leicester), S. D. Barthelmy (GSFC), et al.
Magnitudes were estimated with the UCAC4 cat. and
are not corrected for galactic dust extinction.
Reference:
http://www.osservatoriobassano.org/GRB.asp
The message may be cited.

with an uncertainty of 1.6 arcsec (radius, 90% confidence).
This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).
This circular was automatically generated, and is an official product of the
Swift-XRT team.

GCN Circular #19984
A. A. Breeveld (UCL-MSSL) and P. A. Evans (U. Leicester)
report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 161004A
85 s after the BAT trigger (Evans et al., GCN Circ. 19979).
A source consistent with the enhanced XRT position (Beardmore et al.,
GCN Circ. 19983) is detected in the initial UVOT exposures in the white
and u filters only.
The preliminary UVOT position is:

If the light curve continues to decay with a power-law decay index of
1.3, the count rate at T+24 hours will be 1.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.9 x
10^-14 (5.9 x 10^-14) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00715084.
This circular is an official product of the Swift-XRT team.

GCN Circular #19986
A. Moskvitin (SAO RAS), A. Volnova (IKI), and A. Pozanenko (IKI)
report on behalf of the larger GRB follow-up team.
We observed the field of GRB 161004A (Evans et al., GCN 19979)
with Zeiss-1000 - the 1-meter telescope of Special Astrophysical
Observatory of Russian Academy of Sciences. We obtain several
images with filter Rc starting on October, 4 (UT) 15:44:35.
In the refined XRT error circle (Beardmore et al., GCN 19983)
we do not detect the optical counterpart (Breeveld and Evans,
GCN 19984). Preliminary photometry of a combined image is following

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.
We thank the staff of the Observatorio Astron=EF=BF=BDmico Nacional in San Pedro
M=EF=BF=BDrtir.

with an uncertainty of 2.2 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 92%.
The mask-weighted light curve shows a single peak starting at T-0.1 sec,
peaking at T+0.5 sec and ending at T+5 sec. T90 (15-350 keV) is 1.08 +- 0.21 sec
(estimated error including systematics).
The time-averaged spectrum from T-0.11 to T+1.06 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
1.26 +- 0.33. The fluence in the 15-150 keV band is 9.5 +- 1.6 x 10^-8 erg/cm2.
The 1-sec peak photon flux measured from T+0.06 sec in the 15-150 keV band
is 1.0 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence
level.
Due to a low signal-to-noise of the data, the lag analysis is not possible.
Therefore, it is not clear whether this burst belongs to a short GRB or not.
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/715084/BA/

GCN Circular #20053
S. D. Barthelmy (GSFC), P.A. Evans (U Leicester), N. Gehrels (GSFC),
A. Y. Lien (GSFC/UMBC), J. P. Norris (BSU), P. O'Brien (U Leicester),
K. L. Page (U Leicester), J. L. Racusin (NASA/GSFC),
and H. Ziaeepour report on behalf of the Swift Team:
We performed further analysis of GRB161004A (trigger #715084;
Evans et al., GCN Circ. 19979), to explore the possibility to
clarify its short/long nature.
Using the full BAT event data set from T-240 to T+962 sec,
the updated T90 is 3 +/- 1.4 s (using the 1-s binned light curve).
This T90 is longer than the 1.08 +- 0.21 s originally reported
(Palmer et al, GCN Circ. 20001). However, we note that due to
the weakness of the burst, the standard pipeline finds a T90 that
varies from ~ 1.3 s to ~ 3 s when using light curves with different
bin sizes.
The image analysis with the BAT data in 15-150 keV finds no significant
detections for extended emission within several hundred seconds
after the trigger. However, the BAT image from T+250 s to T+350 s,
which is the time period around the peak of the XRT flare,
does show a slight increase of significance of 3.8 sigma (comparing
to ~ 1 to 2 sigma detections for other periods). Therefore, it is possible
that the XRT flare might be the second peak of the prompt emission, but
was close to the detection limit of the BAT.
The time averaged spectrum is best fit by a simple power-law model,
with an index of 1.3 +/- 0.33. This value is consistent with the average
value of the short GRB distribution, and slightly on the harder side of the
long burst distribution, but still within the majority (Lien & Sakamoto et al.
2016).
The lag analysis is still unavailable due to the weakness of the burst
(Palmer et al, GCN Circ. 20001).
The bright flare in the XRT light curve around T+300 s is unusual
for short GRBs. However, Swift have detected short GRBs with
flares before (e.g., GRB 070724A, GRB 100816A; Margutti et al., 2011,
MNRAS, 417, 2144), and thus we cannot rule out the short-burst
possibility base on this.
Therefore, we conclude that the burst nature remains ambiguous
with current information.